Thursday, Oct 30, 2014

Earthquake acoustics can predict massive tsunamis

Written by Pti | Washington | Posted: June 7, 2013 4:12 pm

Scientists have developed a new technique that could be applied worldwide to create an early warning system for massive tsunamis triggered by earthquakes.

Scientists from Stanford University have identified key acoustic characteristics of the 2011 Japan earthquake that indicated it would cause a large tsunami. The same technique could be used to create an early warning system for massive tsunamis,they believe.

On March 11,2011,a magnitude 9.0 undersea earthquake occurred 64 kms off the shore of Japan. The earthquake generated an unexpectedly massive tsunami that washed over eastern Japan roughly 30 minutes later,killing more than 15,800 people and injuring more than 6,100.

Now,computer simulations by Stanford scientists reveal that sound waves in the ocean produced by the earthquake probably reached land tens of minutes before the tsunami.

If correctly interpreted,they could have offered a warning that a large tsunami was on the way.

Although various systems can detect undersea earthquakes,they can’t reliably tell which will form a tsunami,or predict the size of the wave. There are ocean-based devices that can sense an oncoming tsunami,but they typically provide only a

few minutes of advance warning.

Because the sound from a seismic event will reach land well before the water itself,the researchers suggest that identifying the specific acoustic signature of

tsunami-generating earthquakes could lead to a faster-acting warning system for massive tsunamis.

The earthquake’s epicenter had been traced to the underwater Japan Trench,a subduction zone about 64 kms east of Tohoku,the northeastern region of Japan’s larger island.

Eric Dunham,an assistant professor of geophysics in the School of Earth Sciences,and Jeremy Kozdon,a postdoctoral researcher,began using the cluster of supercomputers at Center for Computational Earth and Environmental Science (CEES) to simulate how the tremors moved through the crust and ocean.

The researchers built a high-resolution model that incorporated the known geologic features of the Japan Trench and used CEES simulations to identify possible earthquake rupture histories compatible with the available data.

The models accurately predicted the seafloor uplift seen in the earthquake,which is directly related to tsunami wave heights,and also simulated sound waves that propagated within the ocean.

The model also generated acoustic data; an interesting revelation of the simulation was that tsunamigenic surface-breaking ruptures,like the 2011 earthquake,produce

higher amplitude ocean acoustic waves than those that do not.

The model showed how those sound waves would have travelled through the water and indicated that they reached shore 15 to 20 minutes before the tsunami.

“We’ve found that there’s a strong correlation between the amplitude of the sound waves and the tsunami wave heights,” Dunham said.

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